1. Field of the Invention
The present invention relates to light sources and particularly to a narrow band, high power and coherent source of light in the red (600-650 nm) spectral region.
2. Background of the Invention
Photodynamic therapy (PDT) is a promising technique for location-specific treatment of cancerous tumors. Its advantages are that the process is localized to the tumor tissue so that relatively little damage occurs to the surrounding healthy tissue, and the procedure can be frequently done without surgery. The PDT technique begins with the administration of a sensitizer drug, known as a photosensitizer either topically, locally or systematically to the patient, followed by the irradiation of the lesion by light which causes selective damage to the tumor tissue. One frequently used photosensitizer is photofrin, which is photo activated with light at 630 nm. Because of a narrow absorption band of photofrin, the light must have a spectral bandwidth no wider than approximately 3 nm. In order to achieve reasonably short treatment times optical powers of greater than 1 W cw are required, and powers of 3-10 W are desirable. Typically red light is delivered to the treated area via multimode optical fiber with a diameter of a few hundred micrometers.
In dentistry, red light is required for cosmetic bleaching of tooth surfaces. The spectral and power requirements (1 W) are less stringent than in PDT. Similarly, large screen visual displays require red light with approximately 1 W cw power, but with near-diffraction limited beam quality, and broad-band spectrum (typically 1-10 nanometers) for speckle-free projection.
At the present time, the only available sources of high power red light are laser diodes or dye lasers pumped by high power argon ion lasers. These systems, however, have serious drawbacks and deficiencies. Although laser diodes operating as short as 630 nm have been demonstrated, they exhibit poor lifetimes and have low output powers. While it might be possible to combine the power of a large number of such diodes through the use of multimode optical fibers, it is difficult to have all of the lasers emit within the narrow 3 nm bandwidth required for efficient photofrin absorption. In addition, the wavelengths of individual diodes can be expected to change with temperature variations and device aging. Similarly, while argon laser pumped dye lasers can generate the required narrow band powers of several watts, the approach suffers from an extremely low electrical-to-optical power conversion efficiency, leading to highly undesirable requirements of large volume water cooling and high voltage power supply lines. Such laser system is also very complex, requiring skilled personnel to maintain proper operation, and very high cost, in the range of $200-300 thousand. Another major drawback is that dye lasers require the use of toxic and hazardous dyes and solvents which have limited lifetimes and which present disposal problems.
The all-solid-state laser system which is the subject of this invention aims at circumventing all of the deficiencies of red emitting lasers just described. Because of its diode pumped solid state configuration, the disclosed laser system does away with the use of dyes, and achieves several orders of magnitude larger electrical-to-optical conversion efficiency than the argon laser based system, allowing operation with a conventional 120 v power supply, and with minimum cooling. In addition, the disclosed laser system is inherently spectrally narrow and capable of maintaining stable operating wavelength. Because of a single spatial mode output, the new laser system also lends itself well toward power scaling through the use of spatial multiplexing, where outputs of many lasers can be efficiently coupled into a multimode power delivery fiber. Finally, as will be described below, the disclosed laser system consist of relatively low cost components and is relatively simple to assemble and align, which will result in a substantially lower overall system cost than existing approaches.